Taurus,
Your mom is trying to reactivate her membership. It is going to cost her gajillions. You need to plainly ASK her point blank why you cannot choose your sorority of choice.
Because the way she is seeing it is like Sistergreek RoyalTempest said: why NOT her organization? Did those girls on campus say ANYTHING to you to think anything otherwise? That is running across her mind...
The reality is she would not be in this situation if she chose to pay for you to college elsewhere... 
Having legacy in your family is a "status symbol" in the NPHC. To say all generations of women are "XYZ Sorority" is important. Now that your folks are empty nesters, they are trying to reconnect to their "activism" roots and friends their age. When one is around all of that, folks are "bragging" about their children's accomplishments. I know, my folks still do it all the time with me and my brother--and we are late 30's/early 40's!!!
I don't think anyone's parents can STOP living vicariously through their kids (33girl
). So, it is what it is...
Only 2 people I know who can answer those kinds of questions, my Soror Barb and Sistergreek Ladygreek. Both have daughters. One person's daughter had chosen to stay in the "family way".
I must say as a huge legacy myself, it is very nice to experience family at various Sorority functions. And that is how I get my paraphrenalia. Also I get a guided level of leadership not just from my Soror mother and grandmother, but also their contemporaries who are actually being my elders to me.
If I had chosen another NPHC, my parents would have harped on their friends to protect me. But, welp, that did not happen. I choose the familiar versus the unknown. It is the best decision in my life.
No one on GC is going to say, join whatever organization. That is a bogus thing to say. Whatever fits for you, is what fits for you. Cool beans. But your mom will eventually forgive overtime. But I doubt it will happen immediately.